Generally, a switching device including an electromagnetically operated device, for example, an electromagnetically operated vacuum circuit breaker includes: a vacuum valve for switching main circuit current; an electromagnetically operated device for driving the vacuum valve; a pressure spring for suppressing electromagnetic repulsion between the contacts caused by a short circuit; an opening spring for increasing the opening speed; and an insulating rod and coupling bar for coupling the electromagnetically operated device to the vacuum valve.
As a requirement of the electromagnetically operated vacuum circuit breaker configured as above, when an overcurrent flows due to a short circuit or the like, the electromagnetically operated device opens the contact of the vacuum valve to interrupt the overcurrent. The electromagnetically operated device is required to perform opening operation immediately after the overcurrent is detected. Furthermore, when the vacuum valve is closed, the electromagnetically operated device is held by magnetic flux of a permanent magnet. When the vacuum valve is to be opened, an opening coil (i.e., drive coil) is energized to cancel the magnetic flux of the permanent magnet, thereby causing the electromagnetically operated device to operate. So, when the holding force (the amount of flux) of the permanent magnet varies due to individual variability, the time from when an opening instruction is received until when the magnetic flux generated by the permanent magnet is canceled varies. Accordingly, the opening operation may vary. As such, if the variation in the holding force of the permanent magnet can be reduced, the variation in the opening operation can also be reduced.
Conventionally, in order to reduce the variation range of the holding force, the residual flux density tolerance or dimensional tolerance of the permanent magnet is reduced. However, correspondingly, the increase in time for adjustment and selection of the magnet result in increase in the cost. As such, if the holding force of the permanent magnet can be easily adjusted, the electromagnetically operated device can be configured at a lower cost.
For example, JP-UM-A-6-86303 (PTL 1) discloses an electromagnet device for overcurrent tripping in which the position of a magnetic material can be adjusted using a screw to divert the magnetic flux and adjust the magnetic attractive force toward a rotary armature.